Conventionally, a heat exchanger has been known, which includes many flat tubes and header collecting pipes connected to the flat tubes and which is configured to exchange heat between refrigerant flowing through each flat tube and air flowing outside the each flat tube. In a heat exchanger disclosed in Patent Document 1, many vertically-extending flat tubes are arranged in the horizontal direction, and a header collecting pipe is connected to a lower end of each flat tube. Moreover, in a heat exchanger disclosed in Patent Document 2, many horizontally-extending flat tubes are arranged in the vertical direction, and a header collecting pipe is connected to an end part of each flat tube.
Refrigerant supplied to the heat exchanger of this type first flows into the header collecting pipe, and then flows so as to branch into the flat tubes. If the heat exchanger of this type functions as an evaporator of a refrigerating apparatus, refrigerant in the two phases of gas and liquid is supplied to the heat exchanger. That is, in this case, the refrigerant in the two phases of gas and liquid is distributed to the flat tubes through the header collecting pipe.
The heat exchanger functioning as the evaporator as in Patent Document 1 is designed such that the mass flow rate of refrigerant flowing into the flat tube is uniformized among the flat tubes. The structure of the heat exchanger disclosed in Patent Document 1 will be described in detail below.
In the heat exchanger of Patent Document 1, a distribution space is formed lateral to an end part of the header collecting pipe, and refrigerant in the two phases of gas and liquid is introduced into the distribution space. In the heat exchanger, an internal space of the header collecting pipe is divided into three chambers arranged in the horizontal direction. Moreover, in the heat exchanger, three distribution paths vertically arranged in line are formed in a partition separating the distribution space and the internal space of the header collecting pipe from each other. Each distribution path is formed for a corresponding one of the chambers of the header collecting pipe. Each distribution path causes a corresponding one of the chambers of the header collecting pipe to communicate with the distribution space. Refrigerant flowing into the distribution space is distributed to each chamber of the header collecting pipe through a corresponding one of the distribution paths, and then flows so as to branch into the flat tubes communicating with the each chamber of the header collecting pipe.
Gravity acts on refrigerant in the two phases of gas and liquid in the distribution space. Thus, as will be seen from paragraph 0018 and FIG. 1 of Patent Document 1, the void fraction for refrigerant increases toward the upper side in the distribution space. That is, in the distribution space, the percentage of low-density gas refrigerant increases toward the upper side, whereas the percentage of high-density liquid refrigerant increases toward the lower side.
In the heat exchanger illustrated in FIG. 1 of Patent Document 1, the number of flat tubes communicating with each chamber of the header collecting pipe is changed in order to equalize the mass flow rate of refrigerant flowing into each flat tube. That is, the number of flat tubes communicating with the chamber corresponding to the uppermost distribution path is the smallest because refrigerant containing much gas refrigerant flows into the uppermost distribution path and the mass flow rate of refrigerant flowing into the chamber corresponding to the uppermost distribution path is relatively low. On the other hand, the number of flat tubes communicating with the chamber corresponding to the lowermost distribution path is the largest because refrigerant containing much liquid refrigerant flows into the lowermost distribution path and the mass flow rate of refrigerant flowing into the chamber corresponding to the lowermost distribution path is relatively high.
In the heat exchanger illustrated in FIG. 5 of Patent Document 1, the diameter of each distribution path is changed in order to equalize the mass flow rate of refrigerant flowing into each flat tube. That is, since refrigerant containing much gas refrigerant flows into the uppermost distribution path, the diameter of the uppermost distribution path is set to the maximum diameter to increase the volumetric flow rate of refrigerant passing through the uppermost distribution path, thereby ensuring the mass flow rate of refrigerant flowing into the chamber corresponding to the uppermost distribution path. On the other hand, since refrigerant containing much liquid refrigerant flows into the lowermost distribution path, the diameter of the lowermost distribution path is set to the minimum diameter to decrease the volumetric flow rate of refrigerant passing through the lowermost distribution path, thereby ensuring the mass flow rate of refrigerant flowing into the chamber corresponding to the lowermost distribution path.